Magnetic force microscopy (MFM) is a development of the noncontact surface analysis type of atomic force microscopy (AFM). MFM is well established as a technique for determining the magnetic fields emanating from magnetic samples, such as thin magnetic films used in magnetic recording media. In an MFM system, a sharp magnetic tip is mounted on a cantilever force sensor which is placed over the surface of the magnetic specimen while the specimen is scanned by a conventional XYZ scanning stage. The magnetic forces that act on the tip from the sample cause a static deflection of the cantilever. These forces are monitored, typically by use of a laser detection system wherein the deflection of the cantilever causes a displacement of a reflected laser light beam. MFM using a magnetized iron tip is described by Martin et al., "High-resolution Magnetic Imaging of Domains in TbFe by Force Microscopy", Appl. Phys. Lett., Vol. 52, No. 3, Jan. 18, 1988, pp. 244-246. The use of silicon tips coated with a film of magnetic material, such as NiFe or CoPtCr, in MFM is described by Grutter et al., "Magnetic Force Microscopy with Batch-fabricated Force Sensors", J. Appl. Phys., Vol. 69, No. 8, Apr. 15, 1991, pp. 5883-5885.
In conventional MFM systems, the probe tip is formed on the end of the cantilever to extend out of the plane of the cantilever in a direction generally perpendicular to the length of the cantilever. Thus, during scanning the cantilever is oriented generally parallel to the sample and the tip extends downward perpendicularly toward the surface of the sample. This perpendicular out-of-plane tip is formed either as a separate structure added to the cantilever end, as shown in U.S. Pat. No. 5,357,787, or as an integral part of the cantilever, as shown in U.S. Pat. Nos. 5,021,364; 5,051,379; and 5,444,244.
Probes with pyramidal tips coated with thin films of magnetic material are the most common type of MFM probes. These types of tips generally have a complex magnetization structure and as a result a complex distribution of magnetic charges due to the pyramidal geometrical shape of the magnetic film. As a result, the lateral resolution of such tips is only moderate. In addition, the stray magnetic field from these types of tips is difficult to calculate, which makes any attempt for quantitative analysis of the acquired MFM data difficult.
Another type of MFM tip, fabricated in an electron beam-induced deposition (EBID) process, is described in IBM's U.S. Pat. No. 5,171,992. This type of tip has a generally conical or needle-like shape and the material forming the tip is essentially a carbon matrix structure of decomposed organic reactants. The magnetic material is then sputtered on the needle-like tip from the side, thus forming a magnetic film on a portion of the generally conical surface of the needle-like tip, with the magnetic charges being localized at either end of the tip. While this type of tip has better performance than a pyramidal tip, it has an unfavorably broad magnetic charge distribution over the tip volume. This is because of the rotational symmetry of the needle-like structure and the round, non-pointed shape of the tip apex, and because of the non-uniform thickness of the magnetic film caused by the geometry of the needle-like shape of the tip.
The use of a focused ion beam (FIB) to machine the tip of an AFM probe is known. For example, a tapping mode FIB machined silicon probe is available from Digital Instruments for use in AFM imaging of samples with deep trenches or steep sidewalls. However, FIB has not been used in the fabrication of MFM probes, and no method of how FIB could be applied to fabricating MFM probes has been suggested.
What is needed is an improved MFM probe with a tip that forms an essentially ideal bar magnet to generate a localized narrow size distribution of magnetic charge close to the sample.